Heat retentive food server

ABSTRACT

The present invention discloses a heat retentive food server for maintaining food at an elevated temperature, especially important in food service operations. The server is a container having an upper shell and lower shell and cavity defined therebetween containing a phase change material. The phase change material is preferably an ethylene acrylate copolymer, which absorbs energy upon heating, and then keeps freshly prepared hot foods placed in the server warm as the phase change material slowly cools and releases heat.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 120 to U.S.Provisional Application No. 60/652,130, filed on Feb. 11, 2005, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to a heatretentive food server for maintaining food at an elevated temperature.The food server uses a phase change material as a heat storage medium.The invention also relates specifically to a thermoplastic, moldable,non-exuding phase change material for use in such food servers.

2. Description of Related Art Several patents and publications are citedin this description in order to more fully describe the state of the artto which this invention pertains. The entire disclosure of each of thesepatents and publications is incorporated by reference herein.

Phase change materials may be repeatedly converted between solid andliquid phases. Their latent heat of fusion can be used to absorb, storeand release heat. These latent heats of fusion are greater than thesensible heat capacities of the materials. For example, in phase changematerials, the amount of energy absorbed upon melting or released uponfreezing is much greater than the amount of energy absorbed or releasedupon increasing or decreasing the temperature of the material over anincrement of 10° C. for example, in the absence of a phase change.

Stated alternatively, upon melting and freezing, a phase change materialabsorbs and releases substantially more energy per unit weight than asensible heat storage material that is heated or cooled over the sametemperature range. The ability of a phase change material to absorb andrelease a large quantity of energy in the vicinity of itsmelting/freezing point is the key to its usefulness in heat retentionuses, e.g., for food servers.

Various food service operations encounter the problem of keeping foodwarm after preparation and for a period of time until consumption. Theseinclude hospitals, nursing homes, hotels, airlines, among others.Numerous devices have been developed to help alleviate the problem offood getting cold. The simplest approach may be a cover or dome with alayer of insulating material disposed therein which can be place over aplate containing the warm food. Insulated bases that can hold a plate orcontainer can be used in conjunction with such covers or domes. Otherdevices include servers which can be heated to an elevated temperature.

Many servers which can be heated to an elevated temperature comprise ahollow shell with a cavity disposed therein. A heat-retentive medium isplaced in the cavity so that when the container is heated, theheat-retentive medium will store heat and then release heat to maintainfood placed adjacent to the server at an elevated temperature. Theheat-retentive medium typically has been a wax or wax mixture, thoughthis requires an additional enclosure within the server to seal the waxso that leakage through the server does not occur. Construction of suchservers is time-consuming and expensive.

Phase change materials have also been used as heat-retentive media withvarying degrees of success. For example, U.S. Pat. No. 5,565,132discloses a multi-component composite for use as a phase change materialto alleviate problems with “oozing” or exuding that are typical of alkylhydrocarbon phase change materials, and to lower the overall cost of thephase change material. The composite comprises a mixture of an alkylhydrocarbon phase change material, a polyolefin resin, an ethylenecopolymer and silica particles. A microwave absorbing additive can alsobe added. The ethylene copolymer component is disclosed to compriseabout 8-12% by weight of the mixture.

U.S. Pat. No. 5,520,103 discloses a phase change material that, whenused as a heat storage medium, eliminates the problem of leakage commonto servers using wax. The phase change temperature of the heat storagemedium is preferably from 190° F. to 230° F. The only heat storagemedium disclosed is ethylene vinyl acetate (EVA). Various food serverconfigurations using a phase change material as the heat-retentivemedium are also disclosed.

One problem with the use of EVA as a heat-retentive medium in foodservers is that over time and with repeated heatings, the EVA degradesand eventually cannot be re-used. This makes the life cycle for suchfood servers relatively short, thereby increasing the cost of use.

A need exists to improve the heat storage medium for heat-retentive foodservers, and more specifically to increase the life cycle of suchservers.

BRIEF SUMMARY OF THE INVENTION

The present invention overcomes the problems associated with the priorart by improving the durability and usefulness of heat retentive foodservers.

In accordance with this concept, the invention herein provides aheat-retentive food server comprising:

-   (a) an upper shell;-   (b) a lower shell;-   said upper shell and said lower shell being joined together and    having a cavity defined therebetween, said upper and lower shells    being comprised of a thermoplastic material, and-   (c) a heat storage material disposed in said cavity, said heat    storage material comprising a phase change material substantially    filling said cavity and being unrestrained therein, whereby said    food server may be heated and placed adjacent to food for    maintaining food at an elevated temperature,-   wherein said phase change material comprises an ethylene acrylate    copolymer.

The invention also provides a heat-retentive food server comprising:

-   (a) an upper shell;-   (b) a lower shell;-   said upper shell and said lower shell being joined together and    having a cavity defined therebetween, said upper and lower shells    being comprised of a thermoplastic material, and-   (c) a heat storage material disposed in said cavity, said heat    storage material comprising a phase change material substantially    filling said cavity and being unrestrained therein, whereby said    food server may be heated and placed adjacent to food for    maintaining food at an elevated temperature,-   wherein said phase change material consists essentially of an    ethylene acrylate copolymer.

Moreover, in another embodiment, the invention provides a material forthermal energy storage comprising an ethylene acrylate copolymer.

Furthermore, the invention provides a method for making a re-heatablefood-warming device comprising the steps of:

-   (a) forming a container from a thermoplastic material wherein said    container has an upper shell and a lower shell;-   (b) injection molding a phase change material comprising an ethylene    acrylate copolymer into an internal cavity formed between the upper    and lower shells such that the phase change material substantially    fills the cavity; and-   (c) sealing the upper and lower shells with a polymeric sealant at    their periphery to contain the phase change material within the    internal cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of a heat retentive food server of the presentinvention.

FIG. 2 shows a view taken through section 2-2 of FIG. 1.

FIG. 3 shows a bottom view of the heat retentive food server of thepresent invention.

FIG. 4 shows the heat retentive food server of FIG. 1, in combinationwith an underlying food plate.

FIG. 5 shows a top view of an additional embodiment of a food server ofthe present invention.

FIG. 6 shows a view taken through section 6-6 of FIG. 5.

FIG. 7 shows a view taken through section 7-7 of FIG. 5.

FIG. 8 shows a section view of an additional embodiment of the presentinvention.

FIG. 9 is a graph depicting the thermal stability of certain ethylenecopolymers under nitrogen.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The following definitions apply to the terms as used throughout thisspecification, unless otherwise limited in specific instances.

As used herein, the term “about” means that amounts, sizes,formulations, parameters, and other quantities and characteristics arenot and need not be exact, but may be approximate and/or larger orsmaller, as desired, reflecting tolerances, conversion factors, roundingoff, measurement error and the like, and other factors known to those ofskill in the art. In general, an amount, size, formulation, parameter orother quantity or characteristic is “about” or “approximate” whether ornot expressly stated to be such.

In this disclosure, the term “copolymer” refers to a polymer polymerizedfrom two or more monomers, and includes terpolymers. The more specificdescription ‘ethylene acrylate copolymer’, ‘ethylene-methyl acrylate(EMA) copolymer’, and the like, is meant to include copolymers which mayalso have a third monomer present.

Copolymers of ethylene and an unsaturated C₁-C₈ alkyl acrylate are wellknown. “Ethylene acrylate copolymers” may also be referred to asethylene-acrylic acid ester copolymers. They can be manufactured fromtwo high-pressure free radical processes: tubular processes or autoclaveprocesses. The difference in ethylene acrylate copolymers made from thetwo processes is described in, e.g., “High flexibility EMA made fromhigh pressure tubular process.” Annual Technical Conference—Society ofPlastics Engineers (2002), 60^(th) (Vol. 2), 1832-1836.

The term “ethylene copolymer”, as used herein, refers to a polymercopolymerized from ethylene and one or more olefinic monomers,preferably one or more alpha-olefin monomers or vinyl acetate monomer.The copolymers of ethylene and alpha olefin can be made by processesemploying Ziegler Natta catalysts or single site catalysts, for example,metallocene catalysts. Copolymers of ethylene and vinyl acetate aretypically produced using free radical polymerization.

Description

In accordance with the present invention and referring now to thedrawings, and more particularly to FIG. 1, there is shown an embodimentof a heat retentive food server. FIG. 1 shows a food cover 1 which maybe used with an underlying food holder 2, such as the plateschematically shown in FIG. 4. The food cover 1 includes an outer shellassembly 10 having an outer surface 11 and inner surface 12. The outerassembly 10 is comprised of an upper shell 15 and a lower shell 20, andhas a cavity 25 defined by the upper and lower shells. As shown in FIG.2, the cavity 25 defined by the upper and lower shells is preferably asingle cavity which is uninterrupted and undivided. In other words,there are no restrictions, partitions or additional enclosures locatedwithin the cavity.

Lower shell 20 has an inner surface 30, an outer surface 32, and a topwall 34. A side wall 38 extends downward from and merges with top wall34. Side wall 38 may also include a lip 42 at a peripheral portion 44,which may also be referred to as the lower end 44 thereof. Lip 42 iscomprised of a substantially horizontal portion 46 and a downwardlyextending portion 48. Lip 42 can engage the rim of a plate or other foodholder placed therebelow as shown in FIG. 4 to completely cover theplate and any food placed thereon.

Upper shell 15 may include a top wall 50 with a side wall or side leg 52extending downwardly therefrom. Upper shell 15 has an inner surface 54,an outer surface 56, and a lower end, or peripheral portion 62. Thus,outer surface 11 of outer shell 10 is comprised of outer surfaces 32 and56 of the lower and upper shells, 15 and 20 respectively. As shown inFIG. 2, the upper shell 15 surrounds the lower shell 20, therebydefining the cavity 25.

The upper shell 15 may also include a central recess 58 and a handle 60.The handle 60 may span the full diameter of the central recess 58. Theupper and lower shells 15 and 20 are joined at their peripheral portionsat a joint 64. FIG. 1 shows a snap-ring configuration at joint 64.However, numerous other configurations are acceptable. For instance, theupper and lower shells 15 and 20 may have flat surfaces at the endsthereof that can simply abut each other as depicted in FIG. 2A. Thejoint 64 may be sealed with a polymeric sealant, for example. Anypolymeric sealant capable of withstanding repeated heating to atemperature of at least about 230° F. without losing its materialproperties may be used. Examples of such sealants include polycarbonatesealants, e.g., 3M® No. 1838 which will withstand temperatures up to350° F.

A heat storage material 70 may be disposed in cavity 25. The heatstorage material 70 comprises, and more preferably is, a phase changematerial as described herein more fully below. The phase change materialsubstantially fills the cavity and has a phase change temperature thatis preferably in the range of about 190° F. to about 230° F. Generally,the phase change material is thermoplastic, moldable and non-exuding.Preferably, it is a solid at room temperature and changes to a viscous,gelatinous state when heated.

The phase change material comprises an ethylene acrylate copolymer, and,preferably, consists essentially of an ethylene acrylate copolymer. Theethylene acrylate copolymer is preferably ethylene methyl acrylate,ethylene ethyl acrylate, ethylene n-butyl acrylate or a combination ofone or more of ethylene methyl acrylate, ethylene ethyl acrylate, andethylene n-butyl acrylate. More preferably, the ethylene acrylatecopolymer comprises or consists essentially of ethylene methyl acrylate.

The heat storage material 70 may contain additional components toenhance or improve certain attributes, or to reduce cost. For example, amicrowave absorbing additive could be included to facilitate heating thephase change material using a microwave oven. Possible additionalcomponents include other polymers, e.g., polyethylene and/or highermelting polyethylenes such as high density polyethylene or low densitypolyethylene, as well as other ethylene copolymers. Generally, the heatstorage material 70, and preferably the phase change material, shouldcontain at least about 12% by weight of an ethylene acrylate copolymer.

Referring again to FIGS. 1 and 2, the upper and lower shells 15 and 20are preferably made from a thermoplastic material capable ofwithstanding repeated prolonged temperature increases to at least about230° F. without a loss of material properties. Any type of thermoplasticmaterial capable of withstanding great temperatures can certainly beused. The heat storage material 70 is used to fill the cavity 25.

The fact that the phase change material is a solid at room temperaturefacilitates construction of the food cover 1 containing the phase changematerial in cavity 25. The phase change material may be formed as ablock in substantially the same shape as the cavity 25. The block can beformed by injection molding or other suitable means to the desiredconfiguration. The upper and lower shells 15 and 20 can then simply bejoined together around the phase change material and sealed with sealant(such as a polycarbonate sealant discussed above) at joint 64.

Alternatively, the upper and lower shells 15 and 20 may be partiallysealed together. The phase change material may be injection molded intothe cavity 25, and then the seal between the upper and lower shells 15and 20 may be completed to contain the phase change material.

An advantage of the phase change material disclosed herein is thatleakage problems are eliminated without the need for other artificialseals, enclosures or restraints in the cavity as required with manyprior art servers. The food server may be made of a simple two-piececonstruction.

When the food server is heated so that the phase change material incavity 25 reaches its phase change temperature (i.e., it changes from asolid to a gelatinous state), heat is stored therein. The phase changematerial will typically change phase when heated above its meltingtemperature for a sufficient time to ensure the phase change iscomplete. The exact time will depend on the particular material and thedesign of the server.

The heat stored in the material 70 is released at a relatively slow rateas the material changes from a gelatinous state back to a solid. Also,the temperature of the material 70 remains relatively constant duringthe phase change. Thus, when the food server 1 is placed over a platecontaining food, heat is directed through the outer shell 10 and willmaintain food placed thereunder at an elevated temperature for anextended period of time.

The underlying food holder may be of virtually any material, e.g.,ceramic or thermoplastic. Typically, food will be placed under the covershortly after it is removed from a cooking apparatus, for example anoven or a steamer, and will be at a temperature in the range of about170° to 200° F. and more preferably from about 185° to 195° F. The foodcover 1 will generally maintain such food at a temperature of at leastabout 145° F. for at least 60 minutes. Thus, when it reaches itsultimate destination, the food is still at an acceptable and desirableserving temperature.

The food server 1 may also include an insulator 80. In the embodimentshown in FIG. 2, the insulator 80 is interposed between the phase changematerial and the upper shell 15. The insulator 80 will prevent heatstored in the phase change material from dissipating rapidly through theupper shell 15. In this way, the heat stored is transferred moreefficiently to the food to maintain the food at an elevated temperaturerather than dissipating through the upper shell 15 and into theenvironment. The insulator 80 can be any type of insulating materialcapable of withstanding the temperatures described herein. Oneacceptable insulating material 80 is a high strength composite papermade from a combination of ceramic fiber, inert fillers and reinforcingfiberglass One such paper is commercially available from the UnifraxCorporation of Niagara Falls, N.Y. (formerly the Carborundum Company)and is known as Fiber Frax® grade 440 ceramic fiber paper.

An additional embodiment of the present invention is shown in FIGS. 5through 7. Referring now to FIG. 5, a food server 100, which comprises afood holder, or food pan, is shown therein. As shown in FIG. 6, the panincludes an outer shell assembly 102 having an inner surface 104 andouter surface 106. The outer shell assembly 102 is comprised of a lowershell 108 and an upper shell 110. The lower shell 108 can be describedas surrounding the upper shell 110. The upper shell 110 has an innersurface 112 and an outer surface 114. The lower shell 108 has an innersurface 116 and an outer surface 118. Thus, inner surface 104 iscomprised of inner surface 112 and inner surface 116 of the upper andlower shells, 110 and 108 respectively. A cavity 120 is defined by theouter shell assembly 102. The cavity is defined by the upper and lowershells 110 and 108 and is preferably a single, undivided anduninterrupted cavity as previously described with respect to theembodiment shown in FIGS. 1 and 2. While the particular embodiment shownin FIG. 5 is rectangular in shape, the server can be of any shape thatis desired, including, but not limited to, shapes such as circular, ovaland square.

The upper shell 110 has a substantially flat base portion for placingfood thereon, i.e., a bottom wall or base 122 and upwardly extendingside wall 124. The bottom wall 122 and upwardly extending side wall 124thereby form a trough or open space for holding food. The upper shell110 may further include an outwardly extending lip 126 at the upper endor peripheral portion 128 of the upper shell. The lip 126 circumscribesthe entire periphery 128 of the upper shell 110.

The lower shell 108 includes a substantially flat support portion, i.e.,a bottom wall or support portion 130, and upwardly extending side walls132. The lower shell 108 surrounds the upper shell 110 as previouslydescribed, and may include an outwardly extending lip 135 at the upperend, or peripheral portion 134 thereof. The upper and lower shells 110and 108 are joined together at their peripheral portions 134 at a joint138, thereby defining cavity 120. The joint 138 is sealed with a sealantsuch as a polycarbonate sealant, discussed above.

A phase change material 136 is disposed in cavity 120. The phase changematerial 136 substantially fills the cavity 120 and is unrestrainedtherein as described with respect to the food cover embodiment. Thephase change material 136 is as described above, comprising an ethyleneacrylate copolymer. Outer shell 102 may likewise be made of thematerials described with respect to the embodiment shown in FIGS. 1 and2.

When the server 100 is heated so that the phase change material 136changes phase to a gelatinous state, food which is typically at atemperature in the range of about 170° F. to 200° F. or more preferably,from about 185° to 195° F. is placed on the food holder 100 after beingremoved from an oven or other heating device, such as boiling water or asteamer. The phase change material 136 will transfer heat through theupper shell 110 to the food so that it is maintained at a temperature ofat least about 145° F. for at least about 60 minutes. Because the foodis in direct contact with the shell 110, the temperature of the foodwill most likely stay above about 145° F. for a period generally longerthan an hour. An insulator 140 may be disposed between the phase changematerial 136 and lower shell 108 so that less or no heat is lost to theenvironment and is instead directed through upper shell 110 to the foodplaced-thereon. The insulator 140 may be made from any suitablematerial, including any material that is described above with respect tothe insulator 80.

An additional embodiment of an upwardly opening food holder is shown inFIG. 8. The configuration shown there, designated by the numeral 200, issimilar to a commonly-known dinner plate and is comprised of an outershell assembly 202 having a cavity 204 disposed therein. Outer shellassembly 202 is comprised of a lower shell 206 and an upper shell 208.The two shells are joined at joint 210 and sealed with a sealant such asa polycarbonate sealant. As described elsewhere herein, the cavity 204will be substantially filled with a phase change material 211 and can beheated so that it will maintain food at an elevated temperature. Aninsulator 212 may be disposed between the lower shell 206 and the phasechange material 211.

As is clear from the foregoing, the phase change material can be formed,by injection molding or otherwise, to match the configuration ofvirtually any cavity shape. A two-piece construction can then be placedaround the formed phase change material and sealed to construct theserver. Alternatively, the phase change material may be injecteddirectly into the two-piece construction.

Thus, regardless of the exact configuration of the food server, theservers of the present invention are of simple construction, eliminateproblems of leakage associated with prior art servers, and effectivelytransfer the heat required to maintain food at a desirable temperaturefor an extended period of time. The inner chambers and other restraintsrequired in previously known servers to eliminate leakage areeliminated.

The invention herein also provides a method for making a re-heatablefood warming device. The first step comprises forming a container bythermoforming or preferably, injection molding, a shell from athermoplastic material. The container preferably has an upper shell anda lower shell and is similar to the food servers described for FIGS. 1-7herein. The upper and lower shells each have an inner surface. Whensealed together, the upper and lower shells define a specificallyconfigured cavity. Alternatively, the shell may be molded in one piece.

In the second step, a phase change material comprising an ethyleneacrylate copolymer is formed to match the size, shape and configurationof the cavity formed between the upper and lower shells. Thus, the phasechange material will substantially fill the cavity formed when the upperand lower shells are sealed together. The phase change material ispreferably injection molded into the cavity. The upper and lower shellsare sealed together, or the seal of a one-piece shell may be completed,in any suitable manner. The seal may be formed or completed at anysuitable time before, during, or after the injection of the phase changematerial. One particularly suitable sealing method is to use a polymericsealant such as a polycarbonate sealant.

Alternatively, the phase change material may be formed separately, forexample when the shell has been molded in two pieces that allow a moldedsolid phase change material to be placed between them. The upper andlower shells are then sealed together in any suitable manner. Oneparticularly suitable method is to use a polymeric sealant such as apolycarbonate sealant.

The resulting food warming device may be heated and reheated and isuseful for holding, or placing adjacent to, freshly prepared hot foodsto maintain the food at an elevated temperature for an extended periodof time.

The present invention can be illustrated by the following examples,which are intended to be exemplary only and are not meant to limit thescope of the invention.

EXAMPLES Examples 1-3, Comparative Example 1

The TGA (Thermogravimetric Analysis) test was run by starting samples at40° C. and raising temperatures at 10° C. per minute and measuring theweight loss of the polymers as they degraded. In this way, ageing of thematerials was accelerated and their relative thermal stabilitydetermined.

After 30 minutes the samples were at 340° C. and after 37 minutes thesamples were at 410° C.

The data demonstrate the fact that EVA (ethylene/18 wt % vinyl acetate,line 1 of FIG. 9) begins to degrade before the more stable E/nBA(ethylene/27 wt % n-butyl acrylate, line 2 of FIG. 9), E/EA (ethylene/18wt % ethyl acrylate, line 3 of FIG. 9), or even better E/MA (ethylene/24wt % methyl acrylate, line 4 of FIG. 9) copolymers. Without wishing tobe held to any theory, it is believed that the greater thermal stabilityof the ethylene acrylate copolymers provides improved durability andusefulness as a heat storage material in a food server, as describedherein.

The data are tabulated in Table I below. TABLE I Thermal Stability ofEthylene Copolymers under Nitrogen Time of Ramp@10° C./m in Example 1Example 2 Example 3 Comparative (start at E/24 wt % E/18 wt % E/27 wt %Example 1 40° C.) MA EA nBA E/18 wt % VA  0 minutes  100%  100%  100% 100% (40° C.) remaining remaining remaining remaining 30 minutes 99.8%99.7% 99.5% 96.5% (340° C.) remaining remaining remaining remaining 37minutes 96.6% 93.2% 91.2% 86.4% (410° C.) remaining remaining remainingremaining

Although preferred embodiments of the invention have been illustrated inthe accompanying drawings and described in the foregoing detaileddescription, it will be understood that the invention is not limited tothe embodiments disclosed, but is capable of numerous modificationswithout departing from the scope and spirit of the invention as definedby the appended claims.

1. A heat-retentive food server comprising: (a) an upper shell; (b) alower shell; said upper shell and said lower shell being joined togetherand having a cavity defined therebetween, said upper and lower shellsbeing comprised of a thermoplastic material, and (c) a heat storagematerial disposed in said cavity, said heat storage material comprisinga phase change material substantially filling said cavity and beingunrestrained therein, whereby said food server may be heated and placedadjacent to food for maintaining food at an elevated temperature,wherein said phase change material comprises an ethylene acrylatecopolymer.
 2. The server of claim 1 wherein said phase change materialcomprises greater than about 12% by weight ethylene acrylate copolymer,based on the total weight of the phase change material.
 3. The server ofclaim 1 wherein said phase change material is a solid at roomtemperature, said phase change material being formed to substantiallymatch the shape of said cavity.
 4. The server of claim 1 where saidphase change material comprises an injection molding having a shapesubstantially matching the shape of said cavity.
 5. The server of claim1, wherein said phase change material changes phase from a solid to aviscous, gelatinous state when heated to an elevated temperature.
 6. Theserver of claim 5, wherein said phase change material changes phase at atemperature of about 190° F. to about 230° F.
 7. The server of claim 1wherein said ethylene acrylate is selected from the group consisting of:ethylene methyl acrylate, ethylene ethyl acrylate and ethylene n-butylacrylate.
 8. The server of claim 1 wherein said heat storage materialadditionally comprises a microwave absorbing additive.
 9. The server ofclaim 1 wherein said heat storage material additionally comprises up to88% of one or more polymers selected from the group consisting ofethylene polymers and ethylene copolymers.
 10. The server of claim 1wherein said phase change material consists essentially of an ethyleneacrylate copolymer.
 11. The server of claim 1 wherein said servercomprises a food cover for use with an underlying food holder, whereinsaid food cover may be heated and placed over said underlying foodholder to maintain food placed upon said holder at an elevatedtemperature.
 12. The food server of claim 11 wherein said food covercomprises said lower shell, and said lower shell comprises (a) a topwall, and (b) a side wall depending downwardly from said top wall, saidupper shell being spaced apart from and surrounding said lower shellthereby defining said cavity.
 13. The food cover of claim 12 furthercomprising an insulator positioned between said heat storage materialand said upper shell so that heat is transferred from said heat storagematerial through said lower shell, and so that a minimum amount of heatis lost through said upper shell, thereby maintaining said food placedunder said lower shell at an elevated temperature for an extended periodof time.
 14. The server of claim 1 wherein said upper shell comprises(a) a substantially flat base portion for placing food thereon; and (b)a side wall extending upwardly from said base portion thereby forming atrough for holding food, said lower shell comprising: (i) asubstantially flat support portion spaced downwardly from said uppershell base portion; (ii) a side wall extending upwardly from said flatportion, said upper and lower shell side walls being joined at the upperends thereof.
 15. The server of claim 15 further comprising an insulatorinterposed between said phase change material and said lower shell, sothat heat is transferred from said phase change material through saidupper shell, and so that a minimum amount of heat is lost through saidlower shell, thereby maintaining said food placed on said upper shell atan elevated temperature for an extended period of time.
 16. A method forrepeatedly storing and releasing thermal energy comprising heating aphase change material so that it undergoes a phase transition and storesthermal energy, and allowing the phase change material to cool so thatit undergoes a second phase change transition and releases thermalenergy, wherein the improvement comprises the phase change materialcomprises an ethylene acrylate copolymer.
 17. The method of claim 16wherein the phase change material comprises greater than 12% by weightof ethylene acrylate copolymer.
 18. The method of claim 16 wherein thephase change material is molded and shaped to fit into the space betweenthe walls of a dual-walled serving container for food or beverages. 19.A method for making a re-heatable food warming device comprising thesteps of: (a) forming a container from a thermoplastic material whereinsaid container has an upper shell and a lower shell; (b) injectionmolding a phase change material comprising an ethylene acrylatecopolymer into an internal cavity formed between the upper and lowershells such that the phase change material substantially fills thecavity; and (c) sealing the upper and lower shells with a polymericsealant at their periphery to contain the phase change material withinthe internal cavity.